It is a known art to calculate a PWM (Pulse Width Modulation) duty ratio (a control signal value) for setting a detected transfer current value at a target transfer current. In this art, the PWM duty ratio is calculated using a relational expression representing relationship between a PWM duty ratio at a time point for generating a required transfer bias voltage and the detected transfer current value that corresponds to the duty ratio at this time point, and thereby generates a given transfer bias voltage.
However, the relationship between the PWM duty ratio and the transfer current value is not necessarily constant over a whole control zone of the PWM duty ratio. Therefore, where it is attempted to calculate the target duty ratio using a single relational expression over the whole control zone of the PWM duty ratio, there is a concern of increasing an error between the calculated target duty value and an actual target duty ratio.
Thus, there is a need in the art for an image forming apparatus having improved accuracy in calculating a control signal value suitable for setting a transfer current at a target value.